In the field of planar-magnetic loudspeakers, the prior art has been primarily made up of what are referred to as double-ended (or double-side driven), and single-ended (or single-side driven) devices, referring to either groups of magnet rows adjacent both surface sides of a thin film diaphragm, in the double-ended case, or magnet rows adjacent just one surface side of the diaphragm, representing a single-ended layout. Examples of both of these approaches are illustrated in, U.S. Pat. No. 3,674,946 “Electromagnetic Transducer” and U.S. Pat. No. 3,919,499 “Planar Speaker” both by James M. Winey, and U.S. Pat. No. 4,037,061 “Planar Pattern Voice Coil Audio Transducer” by Daniel R. von Recklinghausen. Applicant hereby incorporates herein by reference any U.S. patents and U.S. patent applications cited or referred to in this application.
Due to having magnets on both surface sides of the diaphragm, prior art double-ended devices can result in an increased and more confined magnetic field, but in exchange for the greater magnetic force they have had a number of limitations. Those shortcomings include a reduced ability to reproduce high frequencies accurately without linear distortions due to acoustic blockage and cavity effects from magnet structures both behind and in front of the vibratable diaphragm reducing acoustic transparency and causing cavity resonances, which can cause aberrations in the high frequency amplitude response and a low pass filter characteristic that can reduce high frequency bandwidth. Additional structural problems are caused by magnetic repulsion forces between the opposing front and back magnet structures centered over the active region of the diaphragm, particularly when high energy magnets are used, which require extensive bracing and/or heavy frame materials to attempt to offset frame flexing and minimize instabilities of diaphragm tension.
Both single-sided and double-sided prior art devices have a common limitation in that they tend to drive the active portion of the diaphragm with weaker force and/or reduced displacement at the outer most edge of the diaphragm and therefore, diaphragm excursions the center of the diaphragm can be much greater than at the outer portions of the diaphragm, causing both less effective use of diaphragm area, and a dynamic non-linear distortion due to changes in effective diaphragm area relative to diaphragm excursion.
Both single-ended and double-ended, devices also tend to have losses due to end conductor traces needing to be routed outside of the magnetic fields and causing resistive losses and un-driven portions of the diaphragm.
Additional limitations of prior art planar-magnetic transducers relate to reflections and standing waves that are due to film edge termination problems due to under-damped, uncontrolled diaphragm energy near the diaphragm edge termination points.
Also, the strongest flux lines at the outer most portion of the film diaphragm most often have the greatest intensity above or below, rather than in the plane of the film diaphragm such that they don't effectively engage the conductive traces on the diaphragm, and therefore contribute very little to the driving force of the outer portion of the diaphragm. This can result in reduced acoustic output and also in less control of the outer portions of the diaphragm, potentially causing frequency response errors.
Additionally, single-end driven planar magnetic transducers, generally do not have the magnetic force and output capability of a double-ended device. Solutions to the lack of diaphragm control have included mechanical damping of the film surface area and tend to be very lossy and raise the effective moving mass, which may cause further inefficiencies and limited control and utilization of the total diaphragm surface area. Also, as planar magnetic devices are made larger or wider to increase output, they tend to lose dispersion in the upper frequency ranges and in some cases beam the sound forward with overly restrictive directivity.
In an attempt to minimize resonances and interference in acoustic output in double-ended transducers caused by the acoustic opacity of magnets blocking acoustic output and resonances caused by the acoustic cavities, prior art solutions have been attempted, such as using thinner profile magnets adjacent one primary output surface side of the diaphragm in a double ended device, such as illustrated in U.S. Pat. No. 3,922,504 “Electroacoustic Transducer”, by Kenichiro Kishikawa, or reducing magnet count adjacent one surface side of a double-ended device, as illustrated in U.S. Pat. No. 6,934,402 “Planar-Magnetic Speakers with Secondary Magnetic Structure”, by James J. Croft III, et al. These approaches can offset part of the amplitude response problems of double-ended devices but still do not equal a one side, fully open, single-ended device in this regard.
It would be valuable to have a new planar magnetic transducer architecture that can improve planar-magnetic transducers by increasing magnetic field strength derived from both sides of the diaphragm, increasing magnetic force and acoustic output, and linearizing diaphragm mobilization while increasing control of the outer edges of the vibratable diaphragm as an improvement over a single-ended planar magnetic transducer without invoking the acoustical response errors and magnetic repulsion derived frame and diaphragm stability problems of a double-sided drive device.